Recently, the demand for Li-ion batteries (LIBs) has grown exponentially with their applications in large-scale devices such as electric vehicles and energy storage systems (ESS). The energy density of LIBs directly affects the performance of these large devices, including the driving range of electric vehicles and operating time of ESS, making them critically important. Furthermore, frequent fluctuations in fuel prices and increasing air pollution have increased the importance of eco-friendly transportation methods, especially electric vehicles. To meet the rapidly expanding market demand, it is necessary to enhance the energy density of LIBs (Wakihara, 2001; Natarajan and Aravindan, 2018; Kebede et al., 2022).

An LIB system composed of LiCoO₂ and graphite was first developed by Sony in 1991 (Ozawa, 1994). Initially, cathode LiCoO₂ exhibits a layered crystalline structure that allows the lithiation and delithiation of Li ions along a two-dimensional plane. Cathode materials with such layered structure have high Li-ion conductivity during the charge and discharge processes because of strong ionic bonding in the metal oxide (M–O) layers formed, which interact with adjacent M–O layers through strong Coulombic forces, facilitating the insertion and removal of Li ions. Unfortunately, LiCoO₂ undergoes an irreversible phase transition to O1 when more than half of the Li ions are removed, limiting its practical capacity to 150 mAh g^-1, despite its theoretical capacity of 274 mAh g^-1 (Gabrisch et al., 2002; Hirooka et al., 2020). Nevertheless, LiCoO₂ possesses many advantages such as excellent cycle life and ease of synthesis. However, with the expansion of secondary battery markets, there is a problem of increasing cobalt (Co) raw material prices associated with the growing consumption of LiCoO₂. Furthermore, owing to its low reversible capacity, it is not suitable for battery systems that require high energy density, such as electric vehicles.

The limitations of LiCoO₂ have prompted the exploration of the more cost-effective LiMO₂ material based on nickel (Ni). Additionally, Ni-based materials can serve not only in batteries but also for various applications, such as catalysts (particularly in processes like the oxidative coupling of methane), making them easily accessible (Pickering et al., 1992). LiNiO₂, which has a layered structure similar to that of LiCoO₂, is an ideal substitute because of its low cost and high reversible capacity (Kalyani and Kalaiselvi, 2005). Ni is a crucial element for increasing the battery capacity because the oxidation potential of Ni²⁺ is lower than that of Co³⁺. Nevertheless, LiNiO₂ also faces challenges in cycling performance attributed to structural instability during the processes of lithiation and delithiation. The problem can be partially resolved by substituting Ni with Co, Mn, or Al. Consequently, researchers have begun studying lithium nickel-cobalt-manganese oxide (Li[Ni_xCo_yMn_1–x–y]O₂, NCM) to address this issue (Tian et al., 2020; Chen S. et al., 2021).

NCM cathode have garnered significant attention and made progress owing to its layered structure, high capacity, and cost-effectiveness. The first NCM cathode developed was NCM333 (Li [Ni_0.33Co_0.33Mn_0.33] O₂), with an equal molar ratio of Ni, Co, and Mn. NCM333 has a discharge capacity of 150 mAh g^-1 at a charging voltage of 4.3 V, which is similar to that of LiCoO₂ (Wang et al., 2019). Increasing the Ni content effectively enhances the energy density and discharge capacity of NCM cathodes at the same charge voltage. To achieve a higher capacity, the nickel content was gradually increased, leading to the development of materials such as NCM622 (Li[Ni_0.6Co_0.2Mn_0.2]O₂) and NCM721 (Li[Ni_0.7Co_0.2Mn_0.1]O₂), with a nickel content of 60% or more. Furthermore, Ni-rich cathodes, such as NCM811 (Li[Ni_0.8 Co_0.1Mn_0.1]O₂) with a Ni content of over 80%, have been developed. Ni-rich cathodes have gained significant attention and are widely used because they offer higher capacity (>200 mAh g^-1) than LiCoO₂, making them one of the most prominent cathode materials in use today (Liu et al., 2015).

Ni-rich cathodes have had positive effects in terms of energy density owing to their high discharge capacity and relatively high operating voltage (3.6 V). However, as the Ni content increases, battery life deteriorates. Figure 1 illustrates the discharge capacity retention of three different materials with varying nickel content: NCM-622, NCM-811, and NCM-90, at 4.3 V, 4.4 V, and 4.5 V (Kim et al., 2019). As shown in Figure 1A, the initial cycle discharge capacities of these three materials at 4.3 V are 180.3, 200.5, and 212.0 mAh g^-1, respectively. As expected, an increase in the nickel content leads to a higher capacity, with NCM-90 having the highest nickel content and capacity. However, as the charge and discharge cycles were repeated, sharp capacity degradation occurred, and after 100 cycles, the capacity retention was inversely proportional to the nickel content. NCM-90, with the highest Ni content, retained only 89.9% of its initial capacity, whereas NCM-811 and NCM-622 exhibited high capacity retentions of 93.8% and 96.3%, respectively. Notably, at 4.4 V and 4.5 V, increasing the nickel content also led to a shorter cycling life. The three main causes of this reduction in the lifespan of Ni-rich cathodes are electrolyte decomposition, cation mixing, and microcracking.

As shown in Figure 2 (Sun et al., 2018), Ni-rich polycrystalline cathodes are synthesized using the co-precipitation method by preparing a mixed solution of transition metal sulfates, such as NiSO₄·6H₂O, MnSO₄·H₂O, and CoSO₄·7H₂O, in a molar ratio of x:y:z. To achieve uniform precursor production, sodium hydroxide (NaOH) and NH₃·H₂O are appropriately used as precipitants, pH controllers, and complexing agents while pumping the mixed solution into a Continuous Stirred-Tank Reactor (CSTR). CSTR induces the precipitation of (Ni_xMn_yCo_z)(OH)₂.

To synthesize a precursor with a uniform cation distribution, various factors such as pH, stirring speed, and temperature must be considered during the co-precipitation process. In a study conducted by Sun et al., the pH value was adjusted to ∼9, and the circulator bath was maintained at a temperature of 50°C with a stirring speed of approximately 500 rpm to synthesize Ni-rich precursors (Sun et al., 2018). In the work of Gomez-Martin et al., the precursor Ni_0.90Mn_0.05Co_0.05(OH)₂ was manufactured by setting the pH value in the range of 7–10, maintaining the circulator bath at a temperature of 60°C, and using a stirring speed of 1,300 rpm (Gomez-Martin et al., 2022).

Next, excess Li sources, such as LiOH or Li₂CO₃, were added to the co-precipitated (Ni_xMn_yCo_z)(OH)₂ precursor and thoroughly mixed. During heat treatment, Li ions were transported to the secondary particle precursor, resulting in Ni-rich secondary particles. Excess Li was used to compensate for Li loss during sintering. In addition, for Ni-rich synthesis, low-temperature sintering is required to prevent cation mixing. In the study by Hua et al., a mixed material was preheated at 550°C for 6 h and then sintered at 850°C for 12 h to synthesize NCM523 (Hua et al., 2014).

In contrast, Xu et al. used a Li:M molar ratio of 1.05:1 to mix LiOH and Ni_0.8Co_0.1Mn_0.1 precursor, followed by low-temperature heat treatment in an oxygen atmosphere (heating the mixture at 500°C for 5 h and then sintering at 740°C for 10 h) to obtain the final Ni-rich NCM811 (Xu et al., 2019). The excess Li reactant and relatively low temperature (<800°C) synthesis inherent to Ni-rich cathodes often leave residual Li impurities on the surface. Residual Li compounds such as LiOH and Li₂CO₃ can undergo electrochemical decomposition during charging, leading to the generation of carbon dioxide and oxygen gases, which can be problematic (Rahman and Saito, 2007; Robert et al., 2015).

Ni-rich cathodes with relatively low nickel content have been the focus of numerous studies aimed at improving their electrochemical performance. Several strategies have been explored to address the challenges and enhance the longevity and efficiency of these cathodes. One approach is surface coating, which involves coating the surface of the cathode particles with materials such as metal oxides (e.g., Al₂O₃ (Negi et al., 2020), ZrO₂ (Yao et al., 2020), TiO₂ (Qin et al., 2016)), or inorganic non-metallic oxides (e.g., SiO₂ (Khollari et al., 2019)). This coating helps block direct contact with the electrolyte, inhibits side reactions, and improves surface stability. Coating is effective for mitigating issues related to surface degradation. Doping is another effective technique in which metal elements [e.g., Zr (Schipper et al., 2016), Co (Xu et al., 2020), Al (Dixit et al., 2017)] or non-metallic elements [e.g., B (Gao et al., 2021)] are introduced into the cathode particles. The addition of dopants to Ni-rich cathode materials contributes to stabilizing the crystal structure and suppressing the cation mixing effect, thereby enhancing lithium ion diffusion capability (Kim et al., 2021). Combining coating and doping approaches has proven to be effective in addressing issues such as side reactions and cation mixing, thereby ensuring a long lifespan for NCM cathodes. However, as the Ni content increases, especially in Ni-rich cathodes with Ni content exceeding 80%, the effectiveness of these strategies diminishes. This is particularly evident in the inability of coating and doping to effectively suppress microcrack formation, which becomes a dominant factor affecting performance. In such cases, achieving an appropriate balance between energy density and long-term lifespan is crucial for practical use.

Figure 5 presents schematic diagrams of crack-free single-crystal Ni-rich and polycrystalline materials. As shown, the primary contributing factor to the degradation of polycrystalline materials is microcracking induced by volumetric changes. These microcracks propagate with cycling and extend to the particle surface through grain boundary, resulting in particle fracture and exacerbation of electrolyte sub-reactions. In contrast, single crystals exhibit a monocrystalline structure that effectively prevents the initiation of microcracks, even in the presence of volumetric changes during charge and discharge cycles (Fan et al., 2020; Ni et al., 2022). Consequently, single crystals do not experience the structural issues associated with particle fracture due to sub-reactions. Moreover, their monocrystalline nature provides superior electronic conductivity compared with that of their polycrystalline counterparts, naturally leading to an extended operational lifespan.

Single crystals, with their distinct characteristics, offer a far-reaching solution compared to secondary particles by fundamentally addressing the issue of particle fracture, resulting in a remarkable enhancement in performance. In a study by Qian et al., a comparison was drawn between single crystals and secondary particles in NCM622 with relatively low Ni content (Figure 6A) (Qian et al., 2020). In stark contrast to secondary particles formed by the aggregation of smaller primary particles, single crystals can be considerably larger. When assessing NCM622, the capacity loss for single crystals (approximately 1–2 μm in size) and polycrystalline structures (approximately 200–500 nm) was found to be 6% and 48%, respectively.

Single crystals devoid of microcracks exhibit limited performance degradation even in the presence of surface degradation factors such as surface reactions. Fan et al. extended the comparison to Ni-rich cathodes with 83% Ni content. In the case of single crystals, the effective suppression of crack formation resulted in an impressive capacity retention of 84.8% after 600 charge–discharge cycles at a 1 C current rate, as shown in Figure 6B. This figure is significantly higher than the 57.4% observed for secondary particles. Consequently, single crystals demonstrate exceptional structural stability, effectively mitigating microcrack formation even in Ni-rich cathodes with high Ni content, making them ideal solutions for high-capacity and long-life applications (Fan et al., 2020). Furthermore, according to Kong et al., the presence of crystal gaps in polycrystalline materials increases inhomogeneity during cycling, resulting in elevated heat generation. This leads to superior performance in single crystals compared to polycrystalline structures. Therefore, single crystals exhibit significantly better thermal stability at high temperatures (Kong et al., 2022).

The absence of cracks in Ni-rich single-crystal cathodes has led to increased research on synthesis methods in recent years. Furthermore, owing to the instability of Ni³⁺ ions, synthesizing single-crystal Ni-rich cathodes with a high nickel content, such as LiNiO₂, is challenging. Consequently, methods with commercial potential, such as solid-state synthesis using precursors and molten salt synthesis, are limited. While there are several other synthesis methods, such as sol–gel (Huang et al., 2011) and spray pyrolysis (Leng et al., 2021), they encounter challenges like complex and immature preparation processes and high synthesis costs, rendering them unsuitable for commercial production. Additionally, methods such as hydrothermal synthesis (Li et al., 2014) have limitations due to the small particle size (<1 µm), and mass production is also challenging. Therefore, in our study, specifically focus on two promising methods: solid-state and melt salt synthesis.

This review aims to comprehensively cover various aspects of single-crystal Ni-rich materials, including their significance, synthesis methods, electrochemical characteristics, and strategies for addressing existing issues. Ni-rich materials with a nickel content of 80% or higher have competitive advantages for high-capacity applications, such as electric vehicles. Polycrystalline Ni-rich materials synthesized using co-precipitation methods have several drawbacks including surface instability, intergranular cracks, and cation mixing. Intergranular cracks, which expose new surfaces and exacerbate surface degradation, are particularly detrimental to the lifespan of these materials. In contrast, single crystals are considered to be an effective and fundamental solution because they do not suffer from microcracks.

There are two primary methods for synthesizing single-crystal Ni-rich materials: solid-state synthesis using precursors, and molten salt synthesis. Solid-state synthesis methods have been widely studied because they allow easy access to precursor materials through commercial co-precipitation, making solid-state synthesis approachable. To achieve highly reactive single crystals with sizes in the range of 2–5 μm, thorough grinding of the precursors is necessary, followed by annealing at an appropriate temperature in the range of 800°C–900°C. However, the high synthesis temperature of solid-state methods can exacerbate cation mixing issues. To address this issue, researchers are exploring molten-salt synthesis methods that leverage eutectic points to lower the synthesis temperature. Nevertheless, an additional step for the thorough removal of residual salts is required for molten salt synthesis.

Even in microcrack-free single crystals, surface modification effectively enhances the first charge-discharge efficiency by suppressing surface degradation, similar to the effect observed in polycrystals. However, current improvements in this regard are not dramatic and remain limited to a few percentage points. Single crystals have particle sizes ranging from 1 to 4 μm, and are significantly larger than the several-hundred-nanometer-sized particles in polycrystals. This difference in particle size results in lower Li diffusion rates within the particles, leading to intraparticle Li diffusion imbalances that significantly reduce the lifespan of these materials. To address this issue, many studies have focused on doping to effectively enhance Li diffusion rates, thus improving the overall lifespan of the material.